1. Field of the Invention
The present invention relates to an apparatus for removing excess developer particles on the surface of a recording element. Such an apparatus finds particular, though not exclusive, application in magnetic printers.
2. Description of the Prior Art
In present-day data procesing equipment, fast printers are finding increasing application, in which the printing of the characters is effected without requiring raised type impacting on a recipient sheet of paper. These printers, called non-impact or strikeless transfer printers, ordinarily comprise a recording element which ususally consists of a rotary drum or an endless belt, on the surface of which sensitized areas can be formed by electrostatic or magnetic means. These sensitized areas are also called latent images and correspond to the characters or images to be printed. These images are then developed, that is to say, made visible, with the aid of a powdery developer deposited on the recording element. This recording element is brought into contact with a sheet of paper so as to permit the developer particles which are retained by sensitized areas of said element to be transferred onto this sheet in order to be fixed definitively thereon.
To apply these solid developer particles to the recording element of a printer of this type, various applicator means can be used, such as, for example, that described and shown as an accessory in U.S. Pat. No. 3,161,544. However, despite all the care taken during the construction of these devices, it is difficult to prevent the developer particles from being deposited not only in excess on the sensitized areas of the recording element, but also, however slightly, outside said areas. This phenomenon is attributed to the fact that the particles, when they recharged with moisture or static electricity, or when they undergo softening which, however, slightly, makes them more or less sticky, adhere to the surface with which they have been brought into contact. The excessive deposit of developer particles on the sensitized areas of the recording element is undesirable because, when this developer is transferred onto the sheet of recipient paper, the developer that has been deposited according to the pattern of the image formed by these sensitized areas may be spread out and, thereby, blurring the image. On the other hand, the deposit of development particles outside the sensitized areas of the recording element itself is undesirable because of the fact that these particles, when they are transferred onto the paper, form a background which reduces the contrast between the transferred image and the original background of the paper.
Various devices have been used in the prior art to eliminate the excess developer on the surface of the recording element. A device of this type is known in which a mass of development particles brought into contact with the surface of the recording element and downstream of the applicator means electrically discharges the particles which adhere to this element outside the sensitized areas, so that these discharged particles are dislodged from the recording element and adhere to this mass. However, the use of such a device has not given complete satisfaction because it does not always ensure a complete electrical discharge of the particles and, therefore, does not ensure the elimination of the developer particles which continue to exist on the recording element outside the sensitized areas thereof. Furthermore, such a device cannot be used to remove the excess particles on the recording element of an electrostatic printer, because this device would risk, when capturing the electric charges of said element, erasing the sensitized areas thereof.
To overcome these drawbacks, a device has been proposed that contains a chamber connected to a pneumatic pressure member and which has opposite the surface of the recording element an opening through which the air sent under pressure into the chamber escapes. This opening is dimensioned such that the escape of air takes place in the form of an air jet which strikes the surface and, thus, blasts away the excess of developer particles that have adhered to said surface. However, this device, although it has given excellent results, still has the drawback that it discharges the particles that have been blasted away by the jet of air into the atmosphere, causing a pollution and always undesirably ending up or coating those who are near the machine.
Another known device operates in the manner of a vacuum cleaner. The excess developer particles are removed without polluting the atmosphere. Such a device has been described and shown in French Pat. No. 2,322,395, which corresponds to U.S. Pat. No. 4,014,065 issued to Hudson on Mar. 29, 1977, and comprises on one side a chamber usually having a cylindrical form and provided with an inlet and outlet opening, each of which is positioned at one of the two ends of the chamber. The outlet opening is connected to a source of partial vacuum and, on the other side, a conduit or supply tube with a constant cross section communications, at one of its ends, with the inlet opening of the chamber. The other end of said conduit has an orifice in the form of a slot positioned at a very short distance from the surface of the recording element. In this device, the orientation of the chamber is such that its generators are orthogonal to the direction of lengthening of the orifice in the form of the slot in the conduit. Moreover, the ratio of the surface of the cross section of the inlet opening to the outlet opening of the chamber is very small, i.e., less than 0.1, which ensures an air flow which is practically uniform throughout the width of the inlet opening of the chamber. It has been observed that in such a device the developer particles which have been drawn in were not always carried appropriately through the chamber during their passage, and the accumulated particles ended up on the wall of the chamber causing a considerable reduction in the exhausting capacity of the apparatus in the long run and, hence, a relatively heavy clogging of the conduit.